When bonding dissimilar metals, if the bonding process used is the same as that used for bonding the same materials, then sufficient seam strength cannot be obtained since a weak intermetallic compound is formed. For example, when an aluminum alloy and a steel material are bonded, weak intermetallic compounds having high hardness such as Fe2Al5 or FeAl3 are formed. It is necessary to control such intermetallic compounds so as to secure the seam strength.
However, a fine and rigid oxide coating is formed on a surface of the aluminum alloy. To remove such a coating, it is necessary to provide a large amount of heat at the time of the bonding operation. As such, a thick intermetallic compound layer grows, which may then cause a problem as the strength of the bonded portion becomes weak.
Thus, when using a combination of such dissimilar metallic materials, the dissimilar materials have been traditionally bonded through a mechanical fastening operation by means of a bolt, rivet, etc. However, this results in an increase of weight or cost. For the bonding operation of such dissimilar metals, a friction pressure bonding is utilized in some components. However, such a friction pressure bonding operation is limitedly used in a bonding operation between rotation bodies having a desired symmetry, etc. Thus, its application is quite limited. Bonding operations such as explosion bonding or hot rolling are well known. However, such operations have facility and efficiency problems, making it difficult to broadly apply such operations to conventional dissimilar metals.
Japanese Laid-Open Patent Publication No. (Hei) 4-127973 discloses a method of performing a resistance welding operation with a current conducting time equal to or less than 10 ms. Such a method is performed when a clad material formed of two types of materials that are same as the dissimilar metals is interposed between the dissimilar metallic materials to be bonded. This is so that the same materials are bonded to each other.
However, in this method, three sheets of the plates are required where two sheets of the plates would have been required. When considering actual manufacturing, fixating the clad material is required in addition to inserting the clad material. Further, new equipment should be assembled in an established welding line, thereby causing a cost increase. Moreover, when bonding aluminum and steel, since the aluminum clad steel itself is manufactured by bonding the dissimilar materials to each other, manufacturing conditions become critical. Thus, it is difficult to obtain a cheap clad material with a stable performance.
Japanese Laid-Open Patent Publication No. (Hei) 6-39558 discloses a method of bonding materials comprising plating aluminum alloy having Al content equal to or more than 20 wt % or pure aluminum on a surface of the steel, contacting the steel with the aluminum material such that the aluminum alloy or pure aluminum has a thickness equal to or more than 2 μm, conducting the current by layering such a plating surface on the aluminum material, preferably fusing the plating layer and hardly fusing the steel material side.
This aluminum plating operation requires a large amount of heat in order to destroy a rigid oxide coating on the surface of the steel at the time of bonding the aluminum plating surface and the aluminum material. Thus, a weak intermetallic compound is formed on an interface between the aluminum plate and the steel, increasing the likelihood that the bond will fail.
When resistance welding three or more sheets of dissimilar material, both the dissimilar material interface and the same material interface are present at the same time. When the resistance heating of the dissimilar material interface is less than the resistance heating of the same material interface, a nugget is formed on the same material interface due to the difference in the resistance heating, while hardly forming on the dissimilar material interface. In such a case, in order to obtain sufficient bonding strength in each of the interfaces, it is necessary to provide enough heat for the nugget to have the desired diameter on the dissimilar material interface side. However, when the necessary heat is provided to produce a large enough nugget on the dissimilar material interface, the resistance heating in the same material interface becomes excessive. Thus, the thickness decreases by such a resistance heating in a low melting point side material, i.e., the aluminum material side in the above-mentioned bonding example of the aluminum and the steel.